Advances in Life Sciences, particularly in genomics and proteomics, have greatly increased the potential number of reactions and analyses that must be performed by the biotechnology and pharmaceutical industries. An estimated 30 million tests are required to screen a typical pharmaceutical company's compound library against target receptors. The typical number of tests will increase dramatically as information is gleaned from the sequencing of the human genome. To meet these increasing throughput demands in an economically feasible manner, miniaturization of tests is imperative.
Technological advances are enabling the demonstration and use of microscale chemical/biochemical reactions for performing various types of analyses. Implementation of these reactions at such smaller scales offer economies that are unmatched by conventional approaches. Reduced volumes can lower costs by an order of magnitude but conventional liquid-handling devices fail at the required volumes. Parallel implementation provides even greater advantages as demonstrated by the use of high-density plates for screening and high-density MALDI-TOF plates for mass spectrometry analyses of proteins. The rate-limiting hardware is low volume liquid transfer technology that is robust and scalable for compounds of interest. With growing demand, the development of fluid handling devices adept at manipulating sub-microliter volumes of multiple reagents is needed.
Current systems for handling liquid reagents often employ a “pick and place” technique where a liquid reagent sample from a source plate, usually a microtiter plate, is picked up and placed into another reservoir known as the target plate. This technique is often applied for replicating plates, where scale reduction between the source and the target plates are beneficially realized. Typically, an appropriate volume is aspirated from a source plate and deposited to a target site on a multiple target plate. In this arrangement, reduced sample volumes and sample spacing are required for higher degrees of miniaturization. These liquid handling systems can broadly be categorized into two liquid dispensing types: contact liquid dispensing devices and non-contact liquid dispensing devices.
One such type of contact liquid handling is capillary contact dispensing where physical contact is necessary for fluid transfer of liquid reagents. By way of example, applying a thin, elongated pin tool, the tip of which is dipped into a liquid reagent sample in the source plate, and then maneuvered into physical contact with a substrate surface at the target site of the target plate for deposit of the liquid reagent sample thereon. Through capillary action, a certain amount of liquid will adhere to the tip, and can then be transferred to the target site upon contact.
This approach, however, is inherently volumetrically inaccurate since the amount of fluid adhered to the pin tool surface can vary with each cycle. Moreover, due to “wicking” of the drops, relatively small dispensing volumes, on the order of picoliters, cannot be repetitively attained with the sufficient accuracy required for scaled-down, high throughput screening assays when delivering on dry surfaces. Further, to estimate the delivery volume, several physical properties and parameters must be considered. These include the surface tension of the liquid reagent, the hydraulic state of the substrate surface, the affinity for the substrate surface of the reagent fluid, the affinity for the pin tool surface of the reagent fluid, the momentum of the delivery contact, and the application of biochemical coatings on the substrate surface just to name a few. Another problem associated with this capillary contact dispensing technique is that it is more vulnerable to inadvertent cross-contamination of the tool tip and target sites, especially when manipulating multiple reagents and the target site density is high. Further, fragile biochemical coatings are often employed on the surface of the test sites that can be easily damaged by the tips of the pin tools during depository contact therebetween.
Regarding non-contact type liquid dispensing systems, liquid dispensing is performed without any physical contact between the dispensing device and the targeted substrate surface. Typically these systems include positive displacement, syringe-based liquid handlers, piezoelectric dispensers and solenoid-based dispensers, each technology of which affords their own advantages and disadvantages. Piezoelectric-based systems, for example, are capable of accurate delivery of low volume liquid handling tasks on the order of picoliters. Further, these devices are used with positional-accurate motion control platforms that enables increased test site array density.
While this approach is capable of accurate reagent delivery of low volumes on the order of picoliters, one problem associated with these systems is that dedicated or fixed sample reservoirs are required which are directly fluidly coupled to the dispense orifices of the piezoelectric head. The application of this non-contact technique, however, is labor intensive when sub-microliter volumes of multiple reagents are required. Moreover, volumetric precision, at picoliter levels are in part due to small dispensing orifice diameters that are subject to frequent plugging. The scalability of these systems is also reduced since the small diameter of the orifice significantly limits the volume dispense per pulse.
Solenoid-based actuation for non-contact liquid dispensing, on the other hand, tend to be significantly more versatile and scalable compared to the piezoelectric-based liquid dispenser systems. Using conventional aspiration techniques to draw liquid reagent sample into a flow path or communication passageway (e.g., of a tube) of the system, relatively larger volumes or replicate smaller volumes can be dispensed with high precision by the solenoid.
One problem associated with these designs, however, is that the solenoid base actuator must be positioned in-line with the dispense flow path. Accordingly, the flow of drawn reagent sample through the components of the dispensing actuator can cause detrimental stiction. Ultimately, volumetric delivery imprecision results, as well a prematurely reducing the life of the dispensing actuator.
To address this problem, other compatible system fluids (typically filtered de-ionized water) are applied upstream from the aspirated liquid reagents to eliminate contact of the reagent with the dispensing actuator. This bi-fluid delivery approach has proven successful for dispensing a wide range of repetitive dispensing volumes. However, the aspiration of large overfill volumes is required due to dispersion or dilution effect at the liquid interface between the sample\reagent and the system fluid. This especially holds true with repetitive liquid dispensing where the repetitive actuation of the solenoids causes increased agitation at the fluid interface. As shown in the chart of FIG. 1 (illustrating the measured concentration of the dispensed reagent sample versus the dispense sequence), the measured concentration of the liquid reagent sample significantly degrades after around the 50th to the 60th discharge, although the volumetric accuracy remains constant.
Accordingly, a scaleable, non-contact, liquid handling system and method is desired that provides repetitive, low volume, non-contact liquid dispensing without the degradation in liquid sample\reagent concentration, and with volumetric precision ranging from microliters to nanoliters.